Price reductions and user-friendly options have allowed NDT to warm up to advanced ultrasonic testing technology, including phased array and time of flight diffraction.
This figure shows beam coverage in a single sequence single element TOFD phased array S-scan phased array E-scan. Moving clockwise from the upper left corner in this figure illustrates the coverage options of the technologies. The single element probe in the upper left corner shows the limited region in the single beam available. In the upper right, a TOFD pair is used in transmit-receive mode and the entire weld volume is covered in a single pass. The lower left illustrates the phased array S-scan created by sweeping from 45 degrees to 70 degrees from a single range of elements (only 16 of the 64 elements available are used for this configuration).
In the lower right of this figure, the E-scan (electronic scan) is illustrated. This duplicates the manual raster but fires the elements sequentially at a fixed angle. Illustrated is a double firing whereby 16 elements are fired in a group making a 45 degree angle (blue) and stepped ahead one element at a time for 49 steps and then immediately after a second angle of 60 degrees is made using 16 elements and also stepped along 49 times to duplicate manual motion of rastering the probe forward and back.
In the TOFD and phased array option, the operator need only move the probe parallel to the weld axis to achieve the full volume coverage of the weld. The speed of scanning can be typically 50 to 100 millimeters per second, or in more production-minded terms, 6 meters per minute. Source: Materials Research Institute
Phased array ultrasonic instrumentation has been around since the late 1950s and early 1960s, but was limited to the medical industry in the early days. In the later 1980s and early 1990s, the technology was adopted by several manufacturers in the nondestructive testing (NDT) industry. Early versions were generally large and software was not exactly user-friendly.
By the end of the 1990s the portable phased array (PA) instrument was introduced. Most recently, reductions in the price of portable PA instruments have provided broader access to the technology. As with the previous single-channel ultrasonic instruments, phased array ultrasonic units are digitally controlled and have a variety of menus requiring operator input. But the menus provided by phased array instruments generally require more input data. Both single-channel and phased array units provide pulser voltage and duration controls, A-scan range and delay controls, plus many other identical aspects. But some of the items in phased array units are unique to the technology.
Although they have seen a lengthy development period in the NDT industry, there are still manufacturers unaware that phased arrays are simply another form of ultrasonic inspection. In spite of the lengthy development period, there has been, as with the time of flight diffraction (TOFD) technique, a long delay in incorporating the technology into the industry. Note that TOFD was introduced in the late 1970s.
Much of this lag can be attributed to the standards and codes used by the manufacturing industry. For many decades ultrasonic test methods have suffered an unfair comparison to radiography. Even the acceptance criteria used in many fabrication standards impose that ultrasonic evaluations can only be made with radiography. For example, the Australian Standard for Pipelines AS 2885.2 would require even interpretation to radiographic assessments that are not entirely compatible to ultrasonic assessments such as:
- Slag inclusions require assessment of flaw width, and are not to exceed 2 millimeters.
- Porosity indications are to be rejected if there are individual pores greater than 3 millimeters diameter or an individual pore exceeds 25% of the thinner of the nominal wall thicknesses joined.
- In a cluster of porosity an individual pore may not exceed 2 millimeters diameter.
Clearly slag width is not a dimension that ultrasonic testing (UT) is well configured to provide. Sizing individual pores or pulling out the maximum dimension of a single pore from a cluster are also feats not possible with UT.
But even before an operator gets to analyzing data, some standards have wording that is totally incompatible with the equipment requirements for phased array technology. In particular, the probe design is usually defined with limits on size and markings. For example, some codes limit the dimension of the probe. One standard stipulates the probe must have a dimension between 15 millimeters to 25 millimeters and the exit point and nominal angle must be clearly marked on the probe. But a typical linear phased array probe is constructed with 64 elements, 10 millimeters wide and spaced along the linear axis at a pitch of 0.6 millimeters. This forms a probe 10 millimeters by 38.4 millimeters. Since a phased array probe can be made to steer in a variety of angles and with a variety of aperture sizes, the refracting wedge is indicated with the “incident” angle instead of the refracted angle, and the concept of a single exit point is not used since the beam may have different exit points depending on the programmed values.
Around the world the technology has become increasingly popular for many reasons, which are seen as advantages over manual ultrasonic testing and radiography, including:
- Availability of permanent records.
- No radiation safety concerns.
- No stoppage of work due to radiation exposure times.
- No nasty chemicals to dispose of.
- Speed of line scan techniques, which are faster than manual scanning.
- Overall cost savings to the project.
In view of the safety and economic pressures, the entrenched standards are seen as roadblocks to technical advances and have been noted-but only recently have the code-writing bodies begun to do anything about it.
A simple single vee bevel weld in a 12.5-millimeter wall plate is inspected from one side with a 60-degree E-scan. Signal amplitude is based on colors with blues being lowest amplitude and green to yellow and up to red increasing amplitudes. A flaw can be seen at the root 5 millimeters from the weld centerline. Source: Materials Research Institute
Recent Code Activities
In North America there has been a significant shift in favor of automated ultrasonic testing in general and in particular for phased array technologies in that venue. Prominent in this area has been the American Society for Mechanical Engineers (ASME) Boiler and Pressure Vessel Code and the American Society for Testing and Materials (ASTM).
ASME has long recognized advanced ultrasonic systems and, as early as 1992, introduced a descriptive appendix (Appendix E) on computerized imaging techniques (CITs).
In more recent ASME activities, there has been the inclusion of several mandatory and non-mandatory appendices and code cases to describe both TOFD and phased array techniques and equipment.
In the 2007 edition of the ASME Code Section V Article 4 the following items are addressed:
- Mandatory Appendix III Phased Array Single Fixed Angle with Manual Raster
- Non-mandatory Appendix E Computerized Imaging Techniques (includes PAs)
- Non-mandatory Appendix L TOFD Sizing Demonstration/Dual Probe
- Non-mandatory Appendix N TOFD Interpretation
A few recent “code cases,” or changes to the code that are activated prior to a full edition change and eventually get incorporated into the body of the code, follow:
- CC-2557 Use of Manual Phased Array S-scan Ultrasonic Examination
- CC-2558 Use of Manual Phased Array E-scan Ultrasonic Examination
- A separate code case is presently in the works for phased array encoded linear scanning
This figure illustrates a 48-millimeter thick section scanned using a pair of phased array probes each using an S-scan from 40 degrees to 60 degrees. The images are arranged to indicate that the beam has skipped off the far surface and are skipping back up to the same surface the probe is on. The indicated volume covered includes the region from just below the penetration point of the double vee. In conjunction with the two S-scans the phased array probes were also configured to provide a TOFD scan. All three scans (right facing S-scan, left-facing S-scan and TOFD) are seen here. The vertical cursor in the TOFD image is synchronized with the S-scans such that the slice through that scan point along the weld corresponds to the same positions that the S-scans are made. The ease with which TOFD can be interpreted is seen here. The upper and lower bounds of the tested weld are indicated and the flaws clearly visible between those limits. TOFD data was collected at the same time as the S-scans thereby having no impact on the total scanning time yet the combined data (pulse-echo S-scans and TOFD) significantly increases the reliability of positioning and characterization of the flaws detected. Source: Materials Research Institute
A special code case was issued in 2000 that provided the greatest impetus to phased array and TOFD techniques in the 20 years since their introduction to NDT: ASME Code Case 2235-9, Use of Ultrasonic Examination in Lieu of Radiography.
Code Case 2235 provided the first real opportunity for ultrasonics to replace radiography based on its single greatest engineering capability over radiography-the ability to provide reasonably accurate vertical sizing of flaws. With the ability of ultrasonic testing to provide the through-wall height of a flaw, the use of fracture-mechanics-based acceptance criteria can now be considered. By using the time differences between signals identified in ultrasonic test methods (back scatter in phased array techniques and forward scatter in TOFD techniques) these techniques are the only options capable of satisfying the requirements of Code Case 2235.
ASME, via the Code Case 2235, was the first opportunity for TOFD and phased array technologies to be used within a codified structure. For although descriptive documents had been written to standardize the application of the TOFD technique in Europe (EN-583-6 and BS-7706), without an acceptance criteria to use its capabilities, it was unlikely that an engineer would take advantage of it.
Since the ASME Code Case 2235 initiated the use of fracture-mechanics-based acceptance criteria, several others have followed suit, notably API 620 (Design and Construction of Large, Welded, Low-Pressure Storage Tanks) and API 650 (Welded Steel Tanks for Oil Storage) in the United States and NEN 1822 (Acceptance criteria for the Time of Flight Diffraction inspection technique) in the Netherlands.
In 2004 ASTM published E-2373 Standard Practice for Use of the Ultrasonic Time of Flight Diffraction (TOFD) Technique, and in 2006 it published E-2491 Standard Guide for Evaluating Performance Characteristics of Phased Array Ultrasonic Examination Instruments and Systems.
These documents provided further opportunity for use of the technology. E-2373 provides guidance on the use of TOFD in a more informative way than the ASME document. E-2491 provides instructions on how to assess performance of the instrumentation and probes used in NDT phased array applications.
A new standard is now being developed in ASTM to address the methods by which phased array techniques can be applied to general weld inspections.
The Advantages Illustrated
For decades ultrasonic testing of welds was accomplished by a person moving a probe over a test specimen and directing its beam into the region of interest. Movement was usually by hand but it could be accomplished by holding the probe in a fixture and moving the part under the probe-as in tube testing with immersion bath-or the probe may have been mounted in a mechanical holder and the probe and holder moved via a motorized controller.
With the probe used in the pulse-echo mode, the area of inspection was limited to the beam directly under the probe and in a region near the central axis of the beam, i.e. a region about 3 millimeters to 5 millimeters in diameter along the axis that the beam was directed.
Comparing the old single element to TOFD or phased array will quickly indicate one of the main advantages of TOFD and phased array UT over conventional single element UT.
Data interpretation is facilitated by simple color-coded or grey-scale images. These can be enhanced with overlays of the weld profiles to aid in locating the sources of indications.
Standard weld inspections using advanced ultrasonic techniques, including TOFD and phased array pulse-echo techniques, provide significant benefits. These include safety from radiation; documentation and permanent repeatable records compared to manual ultrasonic tests; speed of inspection when using line scans; and easy interpretation, especially when TOFD and phased array scans are used together.
Now that the major North American codes and standards have started to address the special considerations of TOFD and phased arrays, the advantages these techniques provide should be more readily available for all to benefit. ndtEdward Ginzel is president of the Materials Research Institute (Waterloo, Ontario, Canada). For more information, call (519) 886-5071, e-mail email@example.com or visit www.mri.on.ca.